Guard against nil source project in GitDeduplicationService

What does this MR do and why?

Adds a nil guard to Projects::GitDeduplicationService#pool_can_fetch_from_source?, which previously called pool_repository.source_project.repository without checking for a nil source_project.

Pool repositories can lose their source project: the pool_repositories.source_project_id foreign key is ON DELETE SET NULL, so deleting a source project leaves the pool with a nil source. If this method were reached with such a pool, it would raise NoMethodError.

This is currently unreachable through the only production caller (Projects::GitGarbageCollectWorker → GitDeduplicationService#execute), because source_project? short-circuits to false when source_project is nil. This change is defensive hardening against future callers or reordering, not a fix for an active crash.

References

• Issue (fix 3): https://gitlab.com/gitlab-org/gitlab/-/work_items/616942

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